Such electrically small antennas, that is, with a size substantially smaller than the wavelength of the signals that they receive and transmit, are particularly used in the portable reception of FM radio waves. Hence such an antenna must be able to be integrated in a unit of small dimensions to meet portability constraints.
Now, it is known that an antenna, irrespective of its type or the technology used to realise it, must have a minimum dimension in the order of the wavelength and typically greater than the quarter wavelength to be able to operate correctly.
For FM frequencies, the wavelength is in the order of 3 meters at 100 MHz, the FM radio band spreads out around this value. For example, in France, the FM band ranges from 88 MHz to 108 MHz. In order to obtain an effective reception, whip antennas are generally used on which the orientation and length is adjusted, that is typically 75 cm for a quarter wavelength at 100 MHz, for the best reception. However this type of antenna cannot be used for the portable applications. Use is therefore made of loop type antennas, which are electrically small antennas whose efficiency is generally very poor. This is expressed in the following equation:
  η  =            R      rad                      R        rad            +              R        ohm            where Rrad is the radiation resistance and Rohm is the ohmic loss resistance.
To improve the efficiency, the techniques used consist of increasing the radiation resistances by increasing the volume occupied by the antenna while providing optimum coupling conditions. This is for example shown in Small Antennas, by Harold Wheeler, IEEE Trans. Ant. Propagation, Vol. AP23, July 1975. AP23, July 1975. As soon as the conducting material used for the radiating element has an acceptable conductivity and the dielectric losses are low, the ohmic loss generally remains low in relation to the radiation resistance. This is not the case when the efficiency is low, which is the case for small antennas.